In some cases it is desirable to have uninterrupted two-way communication between two devices. For example, telephone calls typically allow uninterrupted two-way communication to simulate a face-to-face communication. Two-way communication can be achieved by dedicating separate communication media to signals in each direction. In some circumstances using separate media can be expensive or impossible. Full-duplex communications allow two devices to both send and receive signals at the same time on a single communication medium. The problem that arises when the devices transmit and receive simultaneously on a communication medium is that the transmitted signal may interfere with the received signal and prevent accurate reception. One way to achieve full-duplex communications is to allocate different frequency bands to each direction of transmission. Confining the transmissions in each direction to non-overlapping frequency bands prevents the signals from interfering with one another. Band-pass filters may be used to cleanly receive each signal while alternate direction transmission continues. This approach may have some draw-backs in certain circumstances, such as reducing the usable bandwidth on the medium available for transmissions in each direction and thus limiting the rate of data transfer.
Communications signal repeaters are devices used to relay signals between communication nodes that may share access to communication medium, but are still unable to communicate directly with each other because, for example, one node is out of range to reliably receive transmissions from another node. For example, repeaters are often employed in Power Line Communications (PLC) networks. Due to limited bandwidth (e.g., 2-80 MHz) and regulatory limits on radio frequency emissions, digital transmissions over power lines have limited range, typically 1-2 km. In order to propagate signals over longer distances on a power line, digital repeaters are mounted on pole tops at distances corresponding to the range limitations of the power line. Reaching customers located at the extreme end of a power line can require as many as 25 hops between repeaters.
Transmissions requiring several hops can incur significant delay and consume a relatively large amount of available bandwidth because each retransmission of a signal occupies bandwidth on the communication medium. PLC devices typically share the medium via a Carrier Sense Multiple Access—Collision Avoidance (CSMA-CA) mechanism. This is essentially a listen-before-talk scheme. If the medium is busy, a station will wait until the medium is idle before sending any queued data. Transmissions on a CSMA-CA network are broken up into units of limited duration called frames. When a station has data to transmit and detects the medium is idle it will contend for the medium by commencing the transmission of a frame. If no collision occurs, the station will complete transmission of the frame. Upon completion of the frame, the station will relinquish the medium for at least a predetermined period of time to allow other stations to contend for the medium with transmissions of appropriate priority level. After the period of time expires, the station may commence transmission of another frame as needed. Repeaters must contend for the medium in order to retransmit a frame of data that they have received. Repeaters receive one or more complete frames of data and store that data until the repeater is able to successfully contend for the medium and commence retransmission of the data in a new frame or frames. This store and forward method causes an additional delay of at least the frame duration for each repeater hop in the path of a message. Each retransmission along the path also occupies bandwidth on the medium for the entire duration of the frame or frames.